Computer only downloads when mouse is moving

I have an HP Workstation XW 4200. I'm using a Dell optical USB mouse. When using the internet, any browser, the computer only downloads when the mouse is being moved. If I open a web page, it won't load unless the mouse is moving. Nothing happens, just a blank screen, and in the bottom left, it says it's loading. As soon as I move the mouse, the page starts to show up. If I stop moving the mouse, the loading stops. Same with any downloads. The download progress stops if the mouse is idle, but starts again if the mouse is moving. I've never seen this before. Any suggestions?

As this happens in all browsers, we can rule out troublesome add-ons. The most likely cause is that the USB mouse mapped itself to the same port as the modem/router when it was installed. As a test, try using a standard mouse to see if the problem persists. If not, try uninstalling the USB mouse driver/software, restart the computer and reinstall. Hopefully, it'll map itself to another port and the problem will go away.